System for warming pressurized gas

ABSTRACT

A plurality of vessels contains pressurized gas. Each vessel fluidly communicates with an adjacent vessel through a line. A heat exchanger is positioned in a heat conducting relationship with each line. The system includes an exhaust valve communicating with lower pressure. In one embodiment, the vessels communicate in series and only one of the vessels communicates with the exhaust valve. Alternatively, the vessels are arranged in a loop configuration with two of the vessels communicating with the exhaust valve through respective lines each containing a shutoff valve. The two shutoff valves are opened and closed in concert to cause the flow in the system to alternate directions as it is being exhausted. In a third configuration, two vessels communicate through a singular line in accordance with the most basic embodiment, but the vessel communicating with the exhaust valve encloses the other vessel. Heat transfer fins are located in the enclosed vessel, and extend into the enclosing vessel. A heat exchanger is located in a heat conducting relationship with the communicating line to heat the gas flowing from the enclosed vessel, and the fins conduct part of this heat to the gas still inside the enclosed vessel.

This is a divisional application of U.S. application Ser. No.09/516,877, filed Mar. 1, 2000, now U.S. Pat. No. 6,658,855.

BACKGROUND OF THE INVENTION

The present invention relates to warming pressurized gas contained instorage vessels while the gas is being exhausted to lower pressure and,more particularly, using a series of fluidly communicating vesselshaving heat sources located in between them to warm the gas as it isflowing through the storage system while the system is being vented tolower pressure.

The flight control systems of modern aircraft use a flight controlcomputer to generate command signals after interpreting and analyzinginputs from the pilot's controls, air data sensors and other aircraftsystems. The position of aerodynamic control surfaces, the configurationof engine nozzles and inlets, and engine fuel controls are adjustedresponsive to the command signals. The foregoing adjustments are usuallymade using electric or hydraulic actuators. Such computer-controlledsystems are commonly referred to as “fly-by-wire” systems.

Fly-by-wire systems offer significant advantages over non-computerizedsystems. The flight control computer can assist the pilot bycontinuously monitoring and adjusting the aircraft's control surfaces tocompensate for changed flight parameters, for example, changes inairspeed. It can also use the inputs from the pilot's controls togetherwith current aircraft flight conditions to provide optimum performancewhile ensuring that the aircraft remains within its permissible flightenvelope. For example, if the pilot pulls back hard on the controlstick, the computer will command the control surfaces to a maximum “g”pull-up for the current airspeed and altitude.

In conjunction with military aircraft, the flight control computer canbe integrated with offensive or defensive systems to optimally positionthe aircraft for weapon deployment, or to maneuver away from threatsmost effectively. In addition, fly-by-wire systems can be used toaugment the stability of aircraft that have compromised their stabilityto obtain a stealthier shape or increased performance, or have had theirstability reduced due to damage. Such stability augmentation may requirecontinuously dithering the control surfaces. In each of these cases thecomputational capability and rapid reaction rate of the fly-by-wirecontrol system allows the pilot to maintain the aircraft in dynamicallystable flight and to safely maneuver it, whereas the numerous sensoryinputs and split-second response times would probably overwhelm a humanpilot acting without such assistance.

It is essential that modern military aircraft have uninterruptedelectrical and hydraulic power to operate their fly by wire controlsystems, as it can take mere seconds without a correction for such anaircraft to become uncontrollable. It is therefore imperative for suchaircraft to have a backup system to supply electrical and hydraulicpower almost instantaneously in the event of the failure of the primarypower systems. The backup system is designed to provide emergency powerfor a relatively short period, e.g., from one to ten minutes. It isintended to provide the pilot with the opportunity either to remedy theproblem with the primary electrical system, to land the aircraft, or toproperly orient the aircraft to enable him and any other occupants tosafely eject from the aircraft.

The emergency power system uses a turbine to drive an electricalalternator or generator, and a hydraulic pump. The turbine wheel isrotated by expanding gases produced by combustion of a mixture of fueland oxygen in a combustor. The combustion must occur reliably at eventhe highest operating altitude, where the oxygen content of the air isquite low. Accordingly, to ensure the availability of emergencyelectrical power throughout the flight envelope, the oxygen for thecombustor is stored on board in a pressurized vessel containing oxygen,air or oxygen-enriched air.

As the stored gas is exhausted into a lower pressure downstream of theexhaust valve of the pressurized vessel, its temperature decreases as itexpands in accordance with the Joule-Thompson effect. Moreover, thetemperature of the gas remaining in the vessel also decreases as theresult of the polytropic expansion of the contained gas. Due to theforegoing, the total temperature drop in the exhausted gas can besignificant if the ratio between the initial stored gas pressure and thefinal stored gas pressure is large and if discharge occurs quickly. Forexample, a temperature drop of approximately 100° F. has been observedduring a two minute discharge from an initial stored gas pressure of5000 psi to a final stored gas pressure of 1500 psi.

The cooling of the gas is undesirable for several reasons. The lowtemperature inside the storage vessel increases the density of the gastherein. This proportionally increases the mass of gas remaining in thevessel when the vessel pressure becomes approximately equal to thedownstream exhaust pressure and the gas no longer flows out of thevessel. The mass of unusable gas remaining in the vessel thus increasesas the temperature decreases. The necessary quantity of useable gascould nonetheless be stored by simply increasing the number or size ofthe vessels. However, the weight and the space that would be necessaryto store additional vessels of pressurized gas are at a premium.

Moreover, the exhaust valve or downstream flow control valve used tometer and control the exhaust flow from the vessel is intricate and hascritical moving components with tight clearances. A lower temperatureextreme causes greater contraction of these components, proportionallyincreasing the overall differential between their dimensions at the hightemperature extreme occurring before exhaust, and the low temperatureextreme which occurs towards the end of the exhaust interval. This makesthe valve's design and manufacture more difficult and expensive.

Furthermore, low temperatures give rise to the possibility that ice willbe formed from vapor carried in the gas stored in the vessel, and thatthis ice will clog the exhaust valve. Extremely low temperatures requirethe use of special dehumidifying equipment to ensure that the vesselsare filled with gas that is extremely dry, so as to prevent theformation of ice. This support equipment, together with the time andlabor necessary to properly use it, adds to the overall cost of theemergency power system.

However, regardless of the care and cost involved in the designing andmanufacturing exhaust valves to strict tolerances, and attendant tofilling the pressurized vessel with gas of extremely low humidity,decreasing the low temperature extreme of the gas inside the vesselinevitably increases the probability that the exhaust valve will bind orsuffer a metering error. Decreasing the low temperature extreme thusadversely affects the reliability of a component whose performance, whencalled upon, will directly affect the likelihood that the pilot willsuccessfully regain primary power, land the aircraft, or safely ejectfrom a properly oriented aircraft.

A decreased low temperature extreme also causes the system's elastomericseals to become more rigid. This adversely affects their sealingqualities and increases the probability of leakage. As a leak woulddecrease the mass of pressurized gas available for generating emergencypower, the increased probability of leakage occasioned by less elasticseals further degrades the reliability of the backup emergency powersystem.

In addition, to efficiently burn, the liquid fuel must first atomize,then vaporize. As the temperature of the gas mixing with the atomizedfuel decreases, the vaporization of the fuel becomes inhibited. When thegas temperature is sufficiently cold, the fuel will not vaporize and, inan extreme case, may even freeze. Either of the foregoing would preventor delay the ignition of the fuel, and adversely affect the performanceof the turbine.

One solution to the problems outlined herein comprises igniting anincendiary device located inside the vessel to increase the temperatureand pressure therein. More particularly, U.S. Pat. No. 4,965,995 and itsdivisional patent, U.S. Pat. No. 5,070,689, disclose positioning anincendiary device inside the pressurized vessel and a pressure sensor inthe outlet of the vessel. When the pressure drops to a level that isinsufficient to provide the desired flow rate of oxidant to thecombustor, the incendiary device is ignited by the sensor. Alternativelyor conjunctively, a temperature sensing probe may be located within thevessel to ignite the incendiary device when the temperature drops to apredetermined level. The ignition of the incendiary device raises thepressure within the vessel as a result of the explosion of the materialof the device or from the heating of the oxidant within the vessel, orboth.

One drawback to this approach is that it requires storing an incendiarydevice on board the aircraft, where accidental detonation from any oneof several causes could injure personnel, damage the aircraft, ordisable the emergency power system. For this reason, the use ofincendiary devices onboard aircraft is avoided.

Furthermore, the incendiary device must contain a fuel and a quantity ofoxidant such that after all of the fuel is reacted, the oxidantconcentration within the vessel remains almost unchanged from itsoriginal concentration. In addition, the ignition of the incendiarydevice may cause the formation of particulate matter, such as carbonsoot, as a by-product. Unless this possibility can be categoricallydisregarded, a filter must be positioned immediately upstream of theoutlet to avoid clogging the flow control valve located downstream ofthe outlet.

Other approaches have heated the air or oxidant downstream of itsexhaust from the pressurized vessel, and before its being mixed with thefuel. For example, U.S. Pat. No. 4,979,362 at column 4, lines 6-15,discloses a heat exchanger heating oxidant flowing from a pressurizedvessel, then combining the heated oxidant with fuel and introducing themixture into a combustor. U.S. Pat. Nos. 4,777,793 and 4,934,136, thelatter being a division of the former, disclose mixing air comingdirectly from a high pressure tank with air which has been heated by aheat exchanger, then mixing this heated air with fuel and igniting theforegoing mixture in a combustor.

However, in heating the air or oxidant downstream of the exhaust valveof the pressurized vessel, the foregoing solutions ameliorate only theproblem of fuel vaporization being inhibited by mixing the fuel withcold gas. Since the gas contained in the pressurized vessel remainsunaffected, heating the downstream gas does not improve the expulsionefficiency for the gas remaining within the vessel. Furthermore, theforegoing approach fails to solve the problems of the hardening of theelastomeric seals and the contraction of the components of the vessel'sexhaust valve.

As may be seen from the foregoing, there presently exists a need in theart for an apparatus which warms the gas used in an aircraft's emergencypower system, while overcoming the shortcomings, disadvantages andlimitations of the prior art. The present invention fulfills this needin the art.

SUMMARY OF THE INVENTION

A plurality of vessels contains pressurized air or some other oxidant.Each vessel fluidly communicates with an adjacent vessel through a line.Each line is located in a heat conducting relationship with a heatexchanger, respectively. The system includes an exhaust valve thatfluidly communicates with one or more of the vessels.

When emergency power is needed, the exhaust valve is opened so that thepressurized system communicates with a lower downstream pressure. Gasflows from one vessel to another, and ultimately out of the system andinto a combustor, where it is mixed with a fuel and burned. Theexpanding gases produced by the combustion rotate a turbine wheel which,in turn, powers an electric alternator or generator, and a hydraulicpump. As the gas in the system passes through each of the lines, it iswarmed by heat conducted from the respective heat exchanger. Thisincreases the temperature of the gas inside the system, as well as thegas flowing into the combustor.

In one embodiment of the present invention, the vessels communicate inseries and only one of the vessels communicates with the exhaust valve.In a second embodiment, the vessels are arranged in a loop configurationso that the gas can alternately flow in opposing directions. Moreparticularly, two of the vessels fluidly communicate with the exhaustvalve through respective lines. Each line contains a shutoff valve. Thetwo shutoff valves are opened and closed in concert to cause the flow inthe system to alternate directions as it is being exhausted. This mixesthe heated gas thoroughly throughout the system.

In a third configuration, two vessels communicate with each otherthrough a singular line in accordance with the most basic embodiment ofthe present invention, but the vessel communicating with the exhaustvalve encloses the other vessel. Heat transfer fins are located in theenclosed vessel, and extend into the enclosing vessel. A heat exchangeris situated in a heat conducting relationship with the communicatingline to heat the gas flowing from the enclosed vessel into the enclosedvessel, and the fins conduct part of this heat to the gas still insidethe enclosed vessel.

These and other features, aspects and advantages of the presentinvention will become better understood with reference to the followingdrawings, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing showing a warming apparatus of the presentinvention having three vessels fluidly communicating in series;

FIG. 2 is a schematic drawing showing a second warming apparatus of thepresent invention having three vessels communicating in a loopconfiguration; and

FIG. 3 is a schematic drawing showing a third warming apparatus of thepresent invention having an upstream vessel fluidly communicating withand being located inside of a downstream vessel.

DETAILED DESCRIPTION OF THE INVENTION

As schematically shown in FIG. 1, gas warming system 21 is a preferredembodiment of the present invention. System 21 includes vessels 23, 25and 27. Vessel 23 fluidly communicates with vessel 25 through line 29,and vessel 25 fluidly communicates with vessel 27 through line 31. Thereis no valve or other restriction on the fluid communication betweenvessels 23 and 25, or between vessels 25 and 27. Vessel 27 fluidlycommunicates with a combustor (not shown) through exhaust valve 33 anddischarge line 35.

System 21 also includes heat exchangers 37 and 39. Line 29 is located ina heat conducting relationship with heat exchanger 37 so that heat fromheat exchanger 37 can be conducted to the cooler gas flowing throughline 29. Line 31 is located in a heat conducting relationship with heatexchanger 39 to enable heat from heat exchanger 39 to be conducted tothe cooler gas flowing through line 31.

In operation, an oxydizer such as air, oxygen, or a mixture of air andoxygen is stored under pressure in vessels 23, 25 and 27. When emergencyelectrical and hydraulic power is needed, exhaust valve 33 is opened andthe stored gas flows towards the lower pressure of discharge line 35 andthe combustor, where it is mixed with fuel and burned. The expandinggases from this combustion turn a turbine wheel (not shown) which, inturn, powers an electrical generator or alternator, and a hydraulicpump.

When exhaust valve 33 is opened, the gas stored in vessel 27, comprisingone third of the gas contained within system 21, will begin flowingthrough exhaust valve 33 and discharge line 35. The gas in vessel 25will flow through line 31 and, after being warmed by heat exchanger 39,into vessel 27, where it will mix with the gas remaining therein. Thegas in vessel 23 will flow through line 29 and, after being warmed byheat exchanger 37, into vessel 25, where it will mix with the gasremaining therein.

While gas is being vented from vessel 27, the temperature of the gasremaining in vessel 27 will increase, or at least decrease more slowlythan it otherwise would, due to being mixed with gas originating fromvessel 25 which has been warmed by heat exchanger 39. Similarly, thetemperature of the gas in vessel 25 will increase, or decrease moreslowly, because it will be mixed with gas originating from vessel 23which has been warmed by heat exchanger 37.

The temperature of the gas flowing into the combustor will initially bethe same low temperature that would be observed had one large vessel hadbeen used without the benefit of warming system 21 of the presentinvention, and then become increasingly warmer in comparison to gasemanating from one large vessel. This is because of the heat energyadded to the gas by heat exchangers 37 and 39.

Since none the gas inside vessel 23 is warmed by heat exchangers priorto it being expelled, vessel 23 has an expulsion efficiency no betterthan that of the one large vessel of the prior art. The expulsionefficiencies of vessels 25 and 27 of warming system 21 are higher thanwould be the case for one large vessel. As the volume of gas containedby vessel 23 is only one third of the total volume contained by warmingsystem 21, the expulsion efficiency of warming system 21 is greater thanthat of a singular vessel of the prior art containing the combinedvolume of the communicating vessels of warming system 21.

The number of vessels in warming system 21 is a variable, with theminimum number being two. The number of heat exchangers is equal to thenumber of vessels minus one. For a given heat exchanger sizing, theoverall expulsion efficiency of system 21 will increase with the numberof vessels because the volume of gas exposed to heat conduction fromheat exchangers will increase in proportion to the number of vessels.Stated alternatively, the volume of the singular vessel containing gasthat will not be exposed to a heat exchangers decreases as the number ofvessels increases.

Moreover, the overall expulsion efficiency of the system can also beincreased by optimally sizing the relative volumes of the vessels as afunction of the system's thermal properties and the rate of gas flowthrough exhaust valve 33.

As schematically shown in FIG. 2, gas warming system 41 comprises asecond preferred embodiment of the present invention, and includesstorage vessels 43, 45 and 47, and exhaust valve 49. Vessel 43 fluidlycommunicates with vessel 45 through line 51, and with exhaust valve 49through line 53. Vessel 47 fluidly communicates with vessel 45 throughline 55, and with exhaust valve 49 through line 57. Exhaust valve 49fluidly communicates with a combustor (not shown) through discharge line59.

There is no valve or other restriction on the fluid communicationbetween vessels 43 and 45, and between vessels 45 and 47. However,solenoid-controlled shutoff valve 61 is located in line 53 and reliefvalve 63 is located in line 57.

System 41 also includes heat exchangers 65 and 67. Line 51 is located ina heat conducting relationship with heat exchanger 65 to enable heatfrom heat exchanger 65 to be conducted to the cooler gas flowing throughline 51. Line 55 is located in a heat conducting relationship with heatexchanger 67 so that heat from heat exchanger 67 can be conducted to thecooler gas flowing through line 55.

When exhaust valve 49 is opened, the pressurized gas contained withinsystem 41 is vented to the combustor. The flow control devices, shutoffvalve 61 and relief valve 63, operate in concert to allow the gas toalternately flow out of vessels 47 and 43. More particularly, the crackpressure of relief valve 63 is set slightly higher than the pressuredrop across relief valve 63 when exhaust valve 49 and shutoff valve 61are both open. Relief valve 63 thus remains closed when shutoff valve 61is open.

The gas flows clockwise in this configuration, as viewed from FIG. 2.Gas escapes from vessel 43 and flows through shutoff valve 61, exhaustvalve 49, and discharge line 59. Gas from vessel 45 is warmed by heatexchanger 65 and flows into vessel 43. Gas from vessel 47 is warmed byheat exchanger 67 and flows into vessel 45 and on into vessel 43. Aftera brief interval, shutoff valve 61 is closed, causing the pressuredownstream of relief valve 63 to drop and, concomitantly, thedifferential pressure across relief valve 63 to increase and exceed thecrack pressure and open the valve. The flow of gas in system 41 thenbegins to circulate in a counterclockwise direction.

Thus, as shutoff valve 61 is alternatively opened and closed, the flowof gas in system 41 alternates direction between clockwise andcounterclockwise, respectively. Each time the direction is changed, gasis heated by conduction from heat exchangers 65 and 67. In this manner,heated gas is more evenly distributed throughout the system, thusincreasing the expulsion efficiency of each of vessels 43, 45 and 47.

System 41 does not have a vessel upstream of exhaust valve 49 that willsuffer a low expulsion efficiency due to none of the gas containedtherein being warmed, in contradistinction to the farthest upstreamvessel 23 of system 21 in the first embodiment. As a result, system 41may have a higher overall expulsion efficiency than system 21.

The number of vessels in system 41 can vary, with the minimum numberbeing two. The number of heat exchangers is equal to the number ofvessels minus one.

Relief valve 63 could be replaced by a second solenoid-controlledshutoff valve. The foregoing solenoid-controlled shutoff valve would bekept closed when shutoff valve 61 was open, and would be kept open whenshutoff valve 61 was closed. Another alternative embodiment includesreplacing exhaust valve 49, shutoff valve 61, and relief valve 63 with asingle three-way valve communicating with discharge line 59. Thoughslightly more complex than using relief valve 63, the forgoingalternatives would more precisely control the fluid flow in system 41and eliminate any downstream pressure perturbations in discharge line59. Otherwise, system 41 would operate in the same way and to the sameeffect as previously described in conjunction with the use of reliefvalve 63.

As schematically shown in FIG. 3, gas warming system 69 comprises athird preferred embodiment of the present invention, and includespressurized outer vessel 71 enclosing pressurized inner vessel 73. Thetwo vessels fluidly communicate without restriction through line 75.Heat exchanger 77 is located in a heat conducting relationship with line75 to enable heat from heat exchanger 77 to be conducted to the coolergas flowing through line 75.

Heat transfer fins 79 are located inside vessel 73 and are also exposedto the gas in vessel 71 such that heat can be conducted from the gas invessel 71 to the gas contained by vessel 73. Vessel 71 fluidlycommunicates with a combustor (not shown) through line discharge line81. Exhaust valve 83 controls the flow through discharge line 81.

In operation, exhaust valve 83 is opened to allow pressurized gas toflow from system 69. As gas is vented to the combustor, gas is drawnfrom inner vessel 73 into outer vessel 71. The gas from inner vessel 73is heated as it passes adjacent to heat exchanger 77, and thus warms theremaining gas contained in outer vessel 71 as mixing occurs. Thisheating of the gas within outer vessel 71 increases its expulsionefficiency.

Some of the heat of the gas in outer vessel 71 is conducted by fins 79to the cooler gas remaining in inner vessel 73. This heat conductionincreases the expulsion efficiency of inner vessel 73. The use of heattransfer fins 79 in conjunction with the concentric vessel configurationof system 69 serves to increase the expulsion efficiency of the upstreamvessel in comparison to that of the vessel farthest upstream in system21, i.e., vessel 23, thereby improving the overall expulsion efficiencyof system 69 over that of a system 21 having two vessels.

In addition to realizing the benefits attendant to increasing theexpulsion efficiency of the storage system, the warmer gas temperatureobtained by a warming system of the present invention reduces thecontraction of the components used in the exhaust valve, decreases theprobability of ice forming in the exhaust flow, improves the flexibilityof various elastomeric seals, and enhances the volatility of the fuel inthe combustor. The warming system of the present invention achieves theforegoing without introducing the hazard associated with storing anincendiary device on board an aircraft, and without forming andinterpolating particulate matter into the system.

It should be understood, of course, that the foregoing relates topreferred embodiments of the invention and that modifications may bemade without departing from the spirit and scope of the invention as setforth in the following claims.

1. An apparatus for warming a pressurized gas, comprising: a pluralityof vessels for storing a gas wherein said plurality of vessels iscomprised of a first vessel and an exhaust vessel, and wherein saidfirst vessel is located inside of said exhaust vessel; an exhaust valvefor exhausting the pressurized gas from the apparatus; a combustorfluidly communicating with said exhaust valve; heat conduction meansexposed to the gas in said first vessel and the gas in said exhaustvessel, said heat conduction means for conducting heat from said exhaustvessel to said first vessel; communication means for fluidlycommunicating said vessels with each other and with said exhaust valve;and a heat exchanger for heating the gas flowing through saidcommunication means, the heat exchanger being positioned outside of theexhaust vessel.
 2. The gas warming apparatus as defined in claim 1wherein: said apparatus is capable of storing the gas at a systempressure; a downstream pressure fluidly communicating with said exhaustvalve; and said system pressure is greater than the downstream pressure.3. The gas warming apparatus as defined in claim 1 wherein: saidcommunication means is comprised of a line communicating said firstvessel with said exhaust vessel; and said exhaust valve communicateswith said exhaust vessel.
 4. The gas warming apparatus as defined inclaim 1 wherein: said heat conduction means is comprised of a fin. 5.The gas warming apparatus as defined in claim 1 further comprising: atank for storing fuel; and the tank fluidly communicating with thecombustor, whereby the gas is mixed with the fuel in the combustor.
 6. Amethod for warming pressurized gas comprising: storing a gas under asystem pressure in a closed system comprised of a plurality of fluidlycommunicating vessels; wherein said vessels are comprised of a firstvessel and a second vessel, said first vessel being situated within saidsecond vessel; conducting heat from the gas within said second vessel tothe gas within said first vessel by inserting heat conduction means intosaid system so that said heat conduction means are exposed to the gaswithin said first vessel and the gas within said second vessel; forcingthe gas to flow from said first vessel to said second vessel byexhausting the gas from the system to a downstream pressure lower thansaid system pressure and feeding the exhausted gas to a combustor; andheating the gas in a heat exchanger as said gas flows from said firstvessel to said second vessel, the heat exchanger being positionedoutside of the second vessel.
 7. The gas warming method as recited inclaim 6 further comprising mixing the exhausted gas with fuel in acombustor.
 8. The gas warming method as recited in claim 6 furthercomprising forcing the gas to flow from said first vessel into saidsecond vessel before exhausting the gas from the system.
 9. The gaswarming method as recited in claim 6 wherein said heat conduction meanscomprise heat transfer fins.
 10. A method for delivering a gaseousoxidant under pressure to a combustor from a system closed off from thecombustor comprising: conducting heat from the gaseous oxidant within afirst vessel to the gaseous oxidant within a second vessel by insertingheat conduction means into said second vessel so that said heatconduction means are exposed to the gaseous oxidant within said firstvessel and the gaseous oxidant within said second vessel; opening anexhaust valve to open said system and allowing the gaseous oxidantcontained under pressure in said first vessel to flow from said firstvessel through said exhaust valve to a combustor and allowing a gaseousoxidant contained under pressure in said second vessel to flow from saidsecond vessel through a line to said first vessel wherein said secondvessel is situated within said first vessel; and heating said gaseousoxidant flowing in said line from said second vessel to said firstvessel for increasing a temperature of gaseous oxidant flowing in saidline.
 11. The method as recited in claim 10 wherein said heat conductionmeans comprises heat transfer fins.
 12. The method as recited in claim10 wherein said gaseous oxidant is selected from the group consisting ofoxygen, air enriched with oxygen, and air.
 13. The method as recited inclaim 10 wherein said gaseous oxidant passes through said exhaust valveto a lesser pressure.